For many years, electric utilities have experienced a deteriorating load factor due to short term seasonal heating and cooling loads that are coincident with winter and summer weather extremes, which usually coincide with the electric utility's electric demand peak. One effort at addressing this problem has been the use of diurnal energy storage which shifts electric demand from daily peak to the daily off-peak period, thereby shifting the electric load from a peaking unit to a base low unit. This is beneficial to the utility, but it does not do anything to alter the utility's annual load factor. Examples of diurnal energy shifting systems which incorporate thermal storage capabilities are disclosed in U.S. Pat. Nos. 3,262,493; 3,339,629 and 4,237,859.
A more recent concept is the so-called ACES system for load shifting which is described in a publication entitled "Assessment of the Load Management Potential of the Annual Cycle Energy System" prepared by Oak Ridge National Laboratory, Oak Ridge, Tenn. (1984). According to the ACES system, in a residential setting a vapor compression prime mover is used to heat the home on demand during the heating season using rejected heat from the production of ice. The ice is stored and then used during the cooling season to cool the home. While the ACES system provides a electric usage shift more significant than diurnal storage, it is not a solution suitable for many applications.
An area in which energy shifts can produce remarkable results for electric utilities is in the area of agricultural heating and cooling requirements which are normally of short duration, and largely coincident with weather extremes, which usually results in a coincidence with the electric utility's demand peak. For example, in the farming of fresh produce it has been found that postharvest produce cooling extends shelf life, slows quality deterioration and improves profitability. Top quality may be produced in the farmer's field, but the prime vine ripened produce will not bring top dollar during the summer harvest season unless the produce has been cooled to the proper holding temperature as soon as possible. Unfortunately, from the electric utility's standpoint, the demand for this type of postharvest produce cooling coincides with the utility's summer peak load period.
Thus, from the standpoint of the electric utility it would be advantageous to have a long term thermal energy storage system that would assist in shifting seasonal peak load usage to a lower cost option for the utility. However, it is also necessary that the initial start-up cost of such a system, and the long range operating cost, provide an adequate incentive for the end user to install and use the system.